This invention relates to the thermochemical desurfacing of metal bodies, commonly called scarfing. More particularly, this invention comprises a method and apparatus for preventing fin formation along the boundaries of a scarfing cut, and for substantially reducing the amount of smoke discharged into the atmosphere during scarfing.
The common practice in scarfing is to first form a molten metal puddle or "hot spot" on the surface of the workpiece by directing preheat flames at a relatively small area of the surface until it reaches its ignition temperature. Thereafter, a stream of oxygen is directed obliquely against the molten puddle to produce a thermochemical reaction on the metal surface. Relative movement is then produced between the oxygen stream and the workpiece which continues the thermochemical reaction along the metal surface, thereby producing the desired desurfacing along the length of the metal body. During the scarfing reaction, a puddle of molten slag, formed downstream of the scarfing reaction zone, continually precedes the advancing reaction zone along the work surface. This slag puddle preheats the metal surface before it is raised to its ignition temperature by the scarfing oxygen stream. Thus, the oxygen stream has a two-fold purpose: first, to effectuate the thermochemical reaction with the metal, and second, to continually push the molten puddle of metal and slag forward to expose fresh metal for the scarfing reaction.
There are two seemingly unrelated problems associated with conventional scarfing: (1) formation of fins when less than the full surface of the workpiece is scarfed and (2) formation of smoke, regardless of whether the full surface or less than the full surface is scarfed. Surprisingly, the preferred embodiment of the present invention is of substantial benefit minimizing the adverse effects of both of these problems, each of which will be discussed separately.
One problem associated with scarfing less than the full width of a metal surface, a process commonly referred to as "spot scarfing", is the formation of "fins" at the edge of the scarfing pass. The term "fin" as used herein refers to a thin flash or wash of pure or slightly oxidized base metal which is solidly joined to the surface of the metal workpiece at the boundary of a scarfing cut; the fins being undesirable defects on the metal body which must be removed before rolling. Fins may be produced by either or both of two unrelated causes. Fins may be produced directly from the primary reaction zone if molten metal is driven laterally out of said zone by the force of the scarfing oxygen stream, causing the molten metal to adhere to the edge of the scarfing cut where it solidifies in place. This type of fin formation (referred to herein, for purposes of convenience, as "primary" fins) can be prevented by using a specially shaped discharge orifice for the scarfing oxygen stream which gradually diminishes the intensity of the oxygen stream at the ends of the orifice to the point where the stream cannot sustain a scarfing reaction along the boundaries of the scarfing cut, but can oxidize molten metal at such boundaries before it solidifies. Nozzles having orifices of the special type described above that are particularly suited for spot scarfing in individual and gang arrangement are described in my U.S. applications Ser. No. 607,888, filed Aug. 26, 1975 and Ser. No. 607,887, filed Aug. 26, 1975 which were copending with my parent application and are now, respectively, U.S. Pat. Nos. 4,040,871 and 4,013,486, the disclosures of which are incorporated herein by reference.
Fins may also be formed by a so-called "secondary" effect which arises when the molten puddle of slag grows progressively larger ahead of the primary reaction zone to the point where the advancing oxygen stream can no longer move the entire puddle forward, and therefor pushes only the center portion of the puddle forward, thereby forcing some of the molten metal at the edges of the puddle laterally beyond the boundaries of the scarfing cut where it solidifies in an unoxidized state. One problem with which the present disclosure is concerned is a method and apparatus for preventing such "secondary" fin formation.
The use of fluid streams to help remove the slag produced by the scarfing reaction is well known. Thus, for example, U.S. Pat. Nos. 2,873,224 and 3,163,559 to Thompson et al. disclose, respectively, solid and hollow streams of high pressure water directed transversely to skim across the work surface just ahead of the scarfing reaction zone to wash away the slag. U.S. Pat. No. 3,354,002 to Gingerich et al. discloses a plurality of high velocity water jets positioned slightly downstream of the reaction zone, perpendicular to the direction of travel of the scarfing reaction, to trap the slag spray and to wash it off the work surface into a slag chute. While the use of such water jets, positioned transverse to the work surface, commonly referred to as "crossfire, slag-water" jets, is effective for removing slag from the surface of the metal, it is nevertheless incapable of preventing fins. This is due primarily to the fact that cross-fire slag-water jets are positioned on one side of the workpiece to blow molten slag from the near side of the cut into a slag receptacle located along the opposite side of the workpiece. This has the effect of minimizing fin-formation on the near side of the cut, while tending to aggravate the problem of fin-formation on the far side.
The second problem caused by conventional scarfing operations is the large amount of smoke, i.e., fumes or vapors and fine particulate matter formed by the scarfing reaction. Prior art techniques for the collection and removal of smoke from the vicinity of the scarfing reaction zone are relatively cumbersome, requiring an inordinate amount of auxiliary cleaning equipment. In scarfing machine installations where the workpiece moves past a stationary machine, the smoke is normally collected in large stationary hoods and removed through ducts maintained under negative pressure by fans. The trapped smoke is thereafter sent through scrubbers and precipitators for removal of contaminants prior to being vented to the atmosphere. In installations where the scarfing machine moves past a stationary workpiece, traveling ducts are used in place of a stationary smoke hood to collect the smoke. In both cases, however, the equipment required for collecting and cleaning the smoke is relatively large, expensive and difficult to maintain.
In addition to preventing secondary fins and reducing the amount of smoke emitted to the atmosphere, the present invention produces a third beneficial result, namely reduction of noise. It is also apparent from comparing scarfing operations with and without the practice of this invention, that practice of the invention reduces the amount of noise emanating from the scarfing reaction.